3.1. Structure Characterization
To effectively graft VTMS on the filler, polydopamine was coated on the surface of mica to provide more active sites (catechol groups). The chemical composition of mica-PDA XPS was probed by XPS. As shown in Figure 1
a, as compared to mica, mica-PDA exhibits the characteristic N1s peak, indicating the deposition of the polydopamine coating on the filler, which is also confirmed by C1s and N1s core-level spectra. For the curved fitted C1s core-level spectrum of mica-PDA shown in Figure 1
b, there are four peak components with the binding energy at 284.6 eV, 285.5 eV, 286.4 eV, and 288.5 eV, representing the C–H species, C–N species, C-O species, and O-C=O species, respectively [26
]. Meanwhile, for the N1s core-level spectrum of mica-PDA shown in Figure 1
c, two peak components at 398.5 eV and 399.5 eV are attributed to –N= species and –N–H species, respectively, which are formed by the indole groups via structure evolution during dopamine self-polymerization [29
]. Furthermore, as shown in the TGA curves of mica and mica-PDA (Figure 1
d), both mica and mica-PDA show the similar degradation behavior, while the weight loss between 200 and 600 °C for mica-PDA is slightly larger than that of mica due to the additional degradation from polydopamine. At 600 °C, mica and mica-PDA show a weight loss of 2.0% and 3.5%, respectively, indicating that the weight content of polydopamine in mica-PDA is 1.5%. Therefore, both XPS and TGA results confirm that polydopamine was successfully coated on the surface of mica in mica-PDA.
To prepare SR/mica-PDA-VTMS composites, mica-PDA (filler), VTMS (silane coupling agent), and DBPMH (curing agent) were added into SR through mechanical blending, followed by being cured at high temperature. Since catechol group active sites (through self-polymerized dopamine) have been introduced on mica-PDA, the reaction between the catechol groups and VTMS occurs (Sketch 1. reaction A), which results in the immobilization of the silane coupling agent on mica-PDA. At high temperature, the decomposition of DBPMH produces free radicals, which initiate the radical addition reactions between the –CH=CH2
groups on both SR and VTMS (Scheme 1
. reaction B). As a result, mica-PDA filler particles should be chemically combined with the SR macromolecular chains, strengthening the filler–rubber interactions, as shown in Sketch 1.
SEM images of ATH and mica powder are shown in Figure 2
a,b, it can be seen that the ATH particles with a diameter of ~2 μm possess the relative regular structure as compared to the mica plates with a length of ~5 μm. It is easy to distinguish ATH and mica from the shape of the particles even when either of them is embedded in the SR matrix. SEM images of the tensile fracture surfaces for the prepared SR composites are shown in Figure 2
c–h. It is found that the ATH particles’ aggregates are exposed on the fracture surfaces and some voids are observed in the SR/ATH composite (Figure 2
c), indicating the weak adhesive force between SR and ATH. For comparison, SR/ATH-VTMS composite (Figure 2
d) displays fewer aggregates and voids resulting from the improved interfacial interactions between filler and rubber with the assistance of VTMS. A similar comparison is also found between the SR/mica (Figure 2
e) and SR/mica-VTMS composites (Figure 2
f), confirming the enhanced interfacial interactions between filler and rubber by adding VTMS. Nevertheless, there exist more voids and larger cracks between the interfaces for the SR/mica-VTMS composite compared to those for the SR/ATH-VTMS composite. This could be attributed to the following factors: (i) the aspect ratio of mica is larger than that of ATH, which makes it more difficult to be covered by rubber macromolecules; (ii) compared with ATH, mica has fewer active sites on the surface that can react with VTMS, resulting in the poorer silane coupling modification effect. After being coated by polydopamine on the surface of mica, filler dispersion is improved for the SR/mica-PDA composite (Figure 2
g) because of the improved compatibility between the filler and rubber [31
]. However, due to the physical interface, the interactions between filler and rubber are still too weak. Meanwhile, for the SR/mica-PDA-VTMS composite (Figure 2
h), a significant improvement on both filler dispersion and interfacial interactions are confirmed through morphology observation. Such improvement should be attributed from the formation of the chemical binding between filler and rubber, which has been illustrated in Sketch 1.
3.2. Performance of SR Composites
The dielectric constant and dielectric loss of the SR composites are presented in Figure 3
a,b, respectively. Since ATH shows much smaller aspect ratio than mica, the ATH-filled SR composites exhibit the lower dielectric constant and dielectric loss than all the mica-filled SR composites. Both dielectric constant and dielectric loss decrease by incorporating VTMS in either SR/ATH or SR/mica composites. For the four different mica-filled SR composites, both dielectric constant and dielectric loss follows the order of SR/mica-PDA-VTMS < SR/mica-PDA < SR/mica-VTMS < SR/mica, with SR/mica-PDA-VTMS composite showing much lower dielectric loss than the other composites. Such behavior is related to the filler–rubber interactions in the composites, which significantly affect interfacial polarization. Generally, the strong filler–rubber interactions could reduce the interfacial polarization, resulting in the lower dielectric constant and dielectric loss. In particular, SR/mica-PDA composite with physical interfacial interactions shows a slightly lower dielectric constant and dielectric loss than SR/mica-VTMS composite. Such result could be ascribed to the better filler dispersion in the SR/mica-PDA composite, as demonstrated in the abovementioned morphologies observation. Using this promising modification method, the SR/mica-PDA-VTMS composite with low dielectric loss can be obtained, which is comparable to the ATH-filled SR composites.
For the mica-filled SR composites, when being exposed under a high-voltage electric field, the large-aspect-ratio mica plates could act as barriers inhibiting the propagation of electrical treeing [32
], which is beneficial to breakdown strength. However, there exist interfaces between the filler phase and the rubber phase, more compatibility between the two phases would result in fewer defects in the interfaces. The insulation material with more defects would result in more space-charges accumulation inside [33
], and thus decrease the breakdown strength. For mica-filled SR composite, there exist more defects inside compared to ATH-filled SR composite, as illustrated in Figure 2
, which is harmful to the breakdown strength. The interfacial modification could decrease the defects in the composites, thus improving the breakdown strength. Therefore, as shown in the Weibull distribution of breakdown strength in Figure 4
a, the breakdown strength of the mica-filled SR composites follows the order of SR/mica-PDA-VTMS > SR/mica-PDA > SR/mica-VTMS > SR/mica, which is closely related to defects observed in the morphologies of the composites, as shown in Figure 2
e–h. Meanwhile, for the ATH-filled SR composites, there is only little increase in the breakdown strength when incorporating VTMS due to the fewer defects in the SR/ATH composite than in the SR/mica composite. The best breakdown strength of ~31.7 kV/mm is achieved for SR/mica-PDA-VTMS composite, much higher than that of SR/ATH-VTMS composite (~22.1 kV/mm), where the main components have been widely used in the commercial silicone rubber insulator.
Moreover, the mechanical performance of the SR composites are compared in Table 1
and the corresponding stress–strain curves are presented in Figure 4
b. A slight increase in tensile strength of the ATH-filled SR composites is found by adding VTMS (from 2.7 MPa to 3.2 MPa), while the enhancement of tensile strength for mica-filled SR composite is more remarkable (from 2.1 MPa to 4.9 MPa), which might be due to the different filler shapes. As for the SR/mica-PDA composite, it shows a little bit better performance in tensile strength and elongation at break than the SR/mica composite due to the improvement of filler–rubber compatibility but still physical interactions. The best tensile strength of 5.4 MPa is achieved for the SR/mica-PDA-VTMS composite, which should be ascribed from the better filler dispersion and stronger interfacial interactions.